Fast starting circuit for crystal oscillators

ABSTRACT

A method for assisting the oscillating starting process of an electromechanical oscillator ( 10 ) has the following steps: detection of oscillator oscillations ( 1   b   , 1   c ) which occur in the output signal ( 1   a ) from the electromechanical oscillator ( 10 ); generation of an excitation pulse ( 3   b ) on the basis of a detected oscillator oscillation ( 1   c ); and feeding ( 3   a ) of the excitation pulse ( 3   b ) to the electromechanical oscillator ( 10 ).

PRIORITY

This application claims priority from German Patent Application No. DE10 2005 001 684.7, which was filed on Jan. 13, 2005, and is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The invention relates to a method and an apparatus for assisting theoscillation starting process of electromechanical oscillators, inparticular crystal oscillators.

BACKGROUND

In order to interchange data between different communication terminals,an identical clock or oscillator signal is required for the transmissionand reception of data in the subscriber appliances. When interchangingdata via radio links, this is generally a radio-frequency clock oroscillator signal (for example at 2 GHz or more). This clock oroscillator signal must be produced in the various communicationterminals independently of one another, for which reason the individualclock signals must be highly frequency stable, in order to maintainsynchronicity between the subscriber appliances. Thus, in general,electromechanical oscillators, such as oscillating crystals or similarpiezoelectric oscillators, are used to produce the stable-frequencyclock or oscillator signal.

Crystal oscillators have a high power consumption, which is particularlydisadvantageous in battery-powered terminals or terminals which areindependent of the mains system, such as mobile telephones, since thecomparatively high energy consumption shortens the operating time ofsuch terminals which are independent of a mains system. The oscillatingcrystals which define the frequency are therefore not generally operatedpermanently but only at those times at which data is actually beinginterchanged, or when such an interchange is intended to be possible.

If the intention is just to ensure the capability to interchange data(for example during the standby mode of mobile telephones), then thesubscriber appliance is generally briefly switched to reception atregular intervals, for which purpose the radio-frequency clock signal isrequired. In consequence, the crystal oscillator is likewise switched onfor short periods at regular intervals. The radio-frequency clock signalis in this case generally required only for a few milliseconds. However,it must be remembered that the crystal oscillator itself also has anoscillation starting period of several milliseconds.

FIG. 1 illustrates the measured oscillation starting process of a knowncrystal oscillator. The supply voltage is switched on at t=0, and theoscillating crystal has stabilized after about 4 ms. The radio-frequencyclock signal is therefore not available until several milliseconds afterthe oscillating crystal has been switched on. If the radio-frequencyclock signal is likewise required only for a few milliseconds, then theoscillation starting process of the crystal oscillator is lengthened,during which process the radio-frequency clock signal, whose overalloperating time is significant, is not yet available. The overalloperating time and thus the energy consumption of the crystal oscillatorwhen operated for short periods in this way are governed to aconsiderable extent by the duration of the oscillation starting process.

SUMMARY

One object of the invention is, thus, to specify a method and anapparatus for assisting or speeding up the oscillation starting processof crystal oscillators. A further object of the invention is to specifyan oscillator circuit having an apparatus for assisting the oscillationstarting process.

This object can be achieved by a method for assisting the oscillationstarting process of an electromechanical oscillator, comprising afeedback method with the following steps:

a) detecting oscillator oscillations which occur in an output signalfrom the electromechanical oscillator;

b) generating an excitation pulse on the basis of a detected oscillatoroscillation; and

c) feeding of the excitation pulse to the electromechanical oscillator.

The detection of the oscillator oscillations may comprise the followingsteps:

-   -   amplifying oscillator oscillations which occur in the output        signal from the electromechanical oscillator; and    -   generating a clock signal on the basis of the amplified        oscillator oscillations.

The excitation pulse can be generated as a function of an enable signal,with a time at which the enable signal causes the generation of theexcitation pulse being predetermined. The enable signal can be producedwith the aid of the clock signal. A plurality of excitation pulses canbe fed to the electromechanical oscillator, by carrying out the feedbackmethod repeatedly. Groups of excitation pulses can be fed to theelectromechanical oscillator periodically, with each group comprising atleast one excitation pulse. A predetermined number of excitation pulsescan be fed to the electromechanical oscillator within one group ofexcitation pulses, with the excitation pulses in the groups beinggenerated on the basis of successive, detected oscillator oscillations.No excitation pulses can be fed to the electromechanical oscillatorduring the time period between the feeding of the groups of excitationpulses. The oscillator oscillations of the electromechanical oscillatorwhich exceed a threshold value can be detected. The electromechanicaloscillator may comprise a piezoelectric oscillator, in particular acrystal oscillator.

The object can also be achieved by an apparatus for assisting theoscillation starting process of an electromechanical oscillator,comprising a detection unit for detection of oscillator oscillationswhich occur in an output signal from the electromechanical oscillator,and a pulse generator for generation of an excitation pulse on the basisof a detected oscillator oscillation which is fed to theelectromechanical oscillator.

The detection unit can be designed to amplify oscillator oscillationswhich occur in the output signal from the electromechanical oscillator,and to generate a clock signal on the basis of the amplified oscillatoroscillations. The detection unit may comprise an inverter chain and atleast one of the inverters in the inverter chain is operated at thetriple point. The first inverter in the inverter chain may detect thoseoscillator oscillations in the electromechanical oscillator which exceeda threshold value, and this first inverter is in the form of a Schmitttrigger. The pulse generator may comprise a univibrator, and an RCelement can be arranged in the delay path of the univibrator. Theapparatus may further comprise a unit for production of an enablesignal, with the pulse generator being designed to receive the enablesignal and feeding the excitation pulse or pulses to theelectromechanical oscillator as a function of the enable signal, and acounter which uses the clock signal to determine the time or times atwhich the enable signal causes the excitation pulse or pulses to be fedto the electromechanical oscillator. The counter can be operable toproduce the enable signal with the aid of the clock signal. The unit forproduction of the enable signal can be designed to cause a plurality ofexcitation pulses to be fed to the electromechanical oscillator. Theunit for production of the enable signal can be designed to cause groupsof excitation pulses to be fed to the electromechanical oscillatorperiodically, with each group comprising at least one excitation pulse.The unit for production of the enable signal can be designed to cause apredetermined number of excitation pulses to be fed to theelectromechanical oscillator within one group of excitation pulses, withthe excitation pulses in the groups being generated on the basis ofsuccessive detected oscillator oscillations. The unit for production ofthe enable signal can be designed to ensure that no excitation pulsesare fed to the electromechanical oscillator during the time periodsbetween the generation of the groups of excitation pulses. Theelectromechanical oscillator may comprise a piezoelectric oscillator, inparticular a crystal oscillator. An oscillator circuit may comprise suchan apparatus and an oscillator unit with an electromechanicaloscillator. The oscillator unit can be in the form of a Colpitzoscillator.

The method according to the invention for speeding up and assisting theoscillation starting process of an electromechanical oscillatorcomprises a feedback method. In a first step of the feedback method,voltage variations and oscillator oscillations which occur in the outputsignal from the electromechanical oscillator are detected. In a secondstep, a detected oscillator oscillation is used to generate anexcitation pulse, which is fed to the electromechanical oscillator in athird step.

Immediately after the supply voltage for the crystal oscillator has beenswitched on, the crystal oscillator will not yet have been stabilized,but it emits an output signal with voltage variations corresponding tothe self-noise of the oscillating crystal, with noise effects andinitial oscillations contained in it. These voltage variations aredetected in the first step of the feedback method according to theinvention. A detected voltage variation is used to produce an excitationpulse, which is emitted to the electromechanical oscillator. The timewhich passes from detection of a voltage variation to the feeding of theexcitation pulse which is produced from it is in general negligible incomparison to the oscillation duration of the electromechanicaloscillator. The excitation pulses are thus fed to the electromechanicaloscillator instantaneously, on the timescale of the oscillatoroscillation, after the detection of a voltage variation or of anoscillator oscillation in the output signal from the electromechanicaloscillator. The voltage variation which is detected in the first step ofthe feedback method is thus amplified with the correct phase. As soon asvoltage variations can be detected in the output signal from theelectromechanical oscillator, these or the mechanical oscillations ofthe electromechanical oscillator on which they are based aredeliberately amplified by using the instantaneously generated excitationpulses that are fed in to drive additional charges into theelectromechanical oscillator. The oscillation amplitude of the voltagevariation or of the oscillator oscillation of the electromechanicaloscillator is increased in this way.

The method according to the invention deliberately amplifies noiseeffects and initial oscillations of the electromechanical oscillator,and the oscillation amplitude of the electromechanical oscillatorincreases considerably faster than in the case of known methods forswitching on an electromechanical oscillator. In consequence, the fulloscillation amplitude of the electromechanical oscillator, that is tosay the steady state, is reached considerably more quickly, and theoscillation starting process is thus significantly shortened. Theuseable radio-frequency clock or oscillator signal is thus availableconsiderably more quickly after the electromechanical oscillator hasbeen switched on, and the time in which the electromechanical oscillatoris not emitting any useable radio-frequency clock signal even though itis consuming power is reduced. The electromechanical oscillator is thusoperated in a considerably more energy-saving manner, since the overalloperating time of the electromechanical oscillator is reduced. Themethod according to the invention lengthens the possible operating timeof communications terminals which are independent of the mains system.

In one advantageous refinement of the method according to the invention,the detection of the voltage variation or of the oscillator oscillationsin the output signal from the electromechanical oscillator comprises theamplification of oscillator oscillations which occur in the outputsignal from the electromechanical oscillator, and the production of aclock signal on the basis of the amplified oscillator oscillations. Theclock signal may in this case be an analogue or else a digital clocksignal. A clock signal (initially irregular) is thus formed from thenoise and from the initial oscillations of the electromechanicaloscillator, from which excitation pulses are subsequently generated forfeeding to the electromechanical oscillator.

In a further advantageous refinement of the method according to theinvention, the excitation pulses are produced as a function of an enablesignal. The enable signal determines whether an excitation pulse isgenerated on the basis of a detected oscillator oscillation and is thenfed to the electromechanical oscillator, and thus predetermines thetimes at which the generation and feeding of one or more excitationpulse or pulses is caused.

In one preferred refinement, the enable signal is produced with the aidof the clock signal which is emitted from the detection unit. Thecoupling of the enable signal to the clock signal results in the enablesignal being matched to the possibly still irregular clock signal and tothe already existing oscillator oscillations.

In a further advantageous refinement of the method according to theinvention, the feedback method is carried out repeatedly. Inconsequence, a plurality of excitation pulses are produced and are fedto the electromechanical oscillator. The plurality of excitation pulsesare generated on the basis of a plurality of voltage variationsdetected, or oscillator oscillations occurring, in the output signalfrom the electromechanical oscillator, and/or on the basis of the clocksignal which is derived from them. The excitation pulses may begenerated on the basis of successive clock signals or oscillatoroscillations in the output signal from the electromechanical oscillator,or else on the basis of non-successive clock signals, so that anexcitation pulse is not fed to the electromechanical oscillator on thebasis of each detected voltage variation or oscillator oscillation.There are therefore gaps or pauses between such separated, successiveexcitation pulses, with these gaps or pauses covering one or moredetected oscillator oscillations. The enable signal is advantageouslyused to control the production of the excitation pulses.

However, the feeding of an excitation pulse to the electromechanicaloscillator excites not just the intended oscillation frequency but alsoharmonics and other undesirable oscillations. The insertion of pauses orgaps between successive excitation pulses allows the undesirableharmonics to decay, and thus contributes to the electromechanicaloscillator oscillating only at the intended oscillator frequency.Carrying out the feedback method according to the invention repeatedlyresults in the oscillation amplitude of the electromechanical oscillatorbeing additionally amplified, and accordingly makes a contribution tofurther shortening the oscillation starting time.

In a further preferred refinement of the method according to theinvention, groups of excitation pulses with a predetermined number ofexcitation pulses are produced, and are fed to the electromechanicaloscillator. The groups comprise at least one excitation pulse. If eachof the groups comprises a plurality of excitation pulses, then theexcitation pulses in the group are produced on the basis of successive,detected oscillator oscillations or on the basis of separated,non-successive detected oscillator oscillations or clock signals. Onegroup is separated from the previous group and from the subsequent groupby a number of detected oscillator oscillations or clock signals, whichare not used as the basis for generation and feeding of any excitationpulse. In one particularly preferred refinement, the successive groupsof excitation pulses have an identical structure and are separated orspaced apart by an identical number of detected oscillator oscillations,thus resulting in a periodic group structure of excitation pulses.

In a further preferred refinement of the method according to theinvention, the only voltage variations and oscillator oscillations thatoccur in them which are detected and amplified in the output signal fromthe electromagnetic oscillator are those which exceed a predeterminedthreshold value. This threshold value is set, for example, such that thedetected voltage variations or oscillator oscillations on the basis ofwhich the clock signal is generated can, with a specific confidencelevel, no longer be added to the (statistical) self-noise of theelectromechanical oscillator. By way of example, this threshold valuemay be in the range between 2 and 10 mV.

In a further preferred refinement of the method according to theinvention, the electromechanical oscillator comprises a piezoelectricoscillator, in particular a crystal oscillator or an oscillatingcrystal.

The apparatus according to the invention for assisting the oscillationstarting process of an electromechanical oscillator and for carrying outthe method according to the invention comprises a detection unit fordetection of oscillator oscillations which occur in the output signalfrom the electromechanical oscillator and of voltage variations whichare present in the self-noise of the electromechanical oscillator andare caused, for example, by noise effects and initial oscillations, aswell as a pulse generator for production of an excitation pulse on thebasis of a detected oscillator oscillation, which excitation pulse isfed to the electromechanical oscillator. In general, the signal delaytimes through the detection unit and the pulse generator of theapparatus according to the invention are negligible in comparison to theoscillation duration of the electromechanical oscillator, so that theexcitation pulse which is produced on the basis of a detected oscillatoroscillation is fed to the electromechanical oscillator instantaneously,on the timescale of the (slower) oscillator oscillation, and thus withthe correct phase. This results in additional charges being driven intothe crystal, and the oscillation amplitude of the electromechanicaloscillator is increased.

In one particularly preferred refinement of the apparatus according tothe invention, the detection unit amplifies the voltage variations andoscillator oscillations which occur in the output signal from theelectromechanical oscillator, and uses them to generate a clock signal.This clock signal is supplied to the pulse generator. The clock signalthus represents the voltage variations and oscillator oscillationsdetected in the output signal from the electromechanical oscillator, andcan thus be used to produce excitation pulses with the correct phase inthe pulse generator.

In one particularly preferred refinement of the apparatus according tothe invention, the detection unit comprises an inverter chain foramplification of oscillator oscillations which occur in the outputsignals from the electromechanical oscillator, and for production of theclock signal. In a further preferred refinement, at least one of theinverters in the inverter chain, but in particular all of the invertersin the inverter chain, is or are operated at the triple point, that isto say the output of the respective inverter is fed back through a highimpedance to its input. The input and the output of the inverter thusassume the same voltage level in the rest state, with this being betweenthe two logic voltage levels. The inverter is thus in a state in whichit produces the highest gain and can quickly amplify voltage variationswhich occur. This analogue operation of an inverter, in particular of aCMOS inverter, results in a faster amplifier of simple design.

In a further preferred refinement of the apparatus according to theinvention, the first inverter in the inverter chain detects andamplifies only those voltage variations and oscillator oscillations inthe electromechanical oscillator which exceed a predetermined thresholdvalue. For this purpose, the first inverter in the inverter chain ispreferably in the form of a Schmitt trigger. This means that the onlyvoltage variations which are detected and amplified are those for which,for example, there is a specific confidence level that they cannot beadded to the self-noise of the electromechanical oscillator.

In a further preferred refinement of the apparatus according to theinvention, the pulse generator comprises a univibrator. The pulsegenerator receives the clock signal, with the clock signal preferablybeing in digital form. The univibrator comprises two signal paths, viawhich signals are supplied to a logic gate. The input signal or thereceived clock signal is supplied directly to the logic gate via thefirst signal path. There are an (odd) number of inverters in the secondsignal path, the delay path, and these have a specific signal delay time(gate delay time). The signal through the delay path thus reaches thelogic gate only with a certain delay, with the signal delay time by theinverter chain also governing the pulse duration of the pulse which isemitted at the output of the logic gate. Depending on the nature of thelogic gate, a signal pulse is produced at its output when a rising or afalling flank occurs, or on both flanks of the input signal of theunivibrator, that is to say of the received clock signal. In oneparticularly preferred refinement, an RC element which precedes theinverter or inverters is located in the delay path of the univibrator,additionally delays the switching of the inverter or inverters followinga flank in the clock signal, and thus additionally governs the pulseduration at the output of the logic gate.

In a further preferred refinement of the apparatus according to theinvention, the pulse generator is designed to receive an enable signal,and the apparatus according to the invention also comprises a unit forproduction of an enable signal. The pulse generator generates theexcitation pulse or pulses as a function of the received enable signal.The enable signal thus controls whether an excitation pulse is generatedin response to an oscillator oscillation detected in the output signalfrom the electromechanical oscillator or in response to a flankoccurring in the clock signal, and whether this is or is not fed to theelectromechanical oscillator.

In a further preferred refinement, the unit for production of the enablesignal is in the form of a counter, which determines the times at whichthe enable signal causes the generation of the excitation pulse orpulses from the clock signal by the pulse generator.

In one particularly preferred refinement of the apparatus according tothe invention, the counter is designed to produce the enable signal onthe basis of the clock signal which is produced by the detection unit.By way of example, the counter is operated with the aid of the clocksignal.

In one particularly preferred refinement of the apparatus according tothe invention, the electromechanical oscillator comprises apiezoelectric oscillator and is, in particular, in the form of a crystaloscillator or an oscillating crystal.

The oscillator circuit according to the invention comprises anoscillator unit with an electromechanical oscillator, as well as anapparatus according to the invention for assisting the oscillationstarting process of an electromechanical oscillator. The addition of theapparatus according to the invention for assisting the oscillationstarting process of an electromechanical oscillator to the knownoscillator unit ensures that a stable-frequency clock or oscillatorsignal is produced even after an oscillation starting period of theelectromechanical oscillator which is considerably shorter than that inthe prior art.

In one particularly preferred refinement, the oscillator circuit is inthe form of a Colpitz oscillator. The electromechanical oscillator is inthis case the element of the Colpitz oscillator which determines thefrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following textwith reference to drawing figures and data curves, in which:

FIG. 1 shows the measured oscillation starting process of anelectromechanical oscillator according to the prior art;

FIG. 2 shows an exemplary embodiment of the apparatus according to theinvention;

FIG. 3 shows a simulation of the oscillation starting response whenusing the method according to the invention; and

FIG. 4 shows a comparison of the oscillation starting response whichresults from the use of the method according to the invention, with theknown oscillation starting response.

DETAILED DESCRIPTION

FIG. 2 shows the circuit diagram of one exemplary embodiment of theoscillator circuit according to the invention. The oscillator circuitcomprises an oscillator unit 1 with a crystal oscillator or oscillatingcrystal 10. A controlled current source 12 supplies a switchingtransistor 11, whose gate is controlled by the oscillating crystal 10.The controlled current source 12 may be controlled, for example, in sucha way that it emits a higher voltage during the oscillation startingprocess, in order to speed up the oscillation starting process. Astabilized-frequency clock signal is emitted at the connection 1 e ofthe switching transistor. The output signal from the oscillating crystal10 is supplied via the supply line 1 a (XTAL) to a detection unit 2. Apulse generator 3 feeds excitation pulses 3 b with the correct phase viathe supply line 3 a (LOAD PIN) to the oscillator unit 1.

Immediately after the supply voltage for the oscillator unit 1 isswitched on, the oscillating crystal 10 has not yet stabilized, and theoutput signal from the oscillating crystal 10 (on the supply line 1 a)essentially contains the self-noise of the oscillating crystal 10 aswell as noise effects and initial irregular oscillations, which aredetected in the detection unit 2 and are mapped to form a digital clocksignal.

In the present exemplary embodiment, the detection unit 2 for thispurpose contains an inverter chain comprising three inverters 21, 24,27, which are DC-isolated from one another and the oscillator unit 1 bymeans of coupling capacitors 23, 26, 29. The inverters 21, 24, 27 areoperated at the so-called triple point, that is to say the output of theinverters 21, 24, 27 is fed back via resistors 22, 25, 28 with a highimpedance to the respective input. High-impedance transistors can alsobe used in each case, instead of the resistors 22, 25, 28. Thehigh-impedance feedback 22, 25, 28 of the individual inverters 21, 24,27 is designed in such a way that, in the rest state, the voltage at theinput and output of an inverter 21, 24, 27 is identical. The resistancevalue is typically 200 kΩ. The voltage at the input and output is inthis case in the middle between the two logic voltage levels, that is tosay, if the two logic voltage levels are, for example, 0 and 2.5 V, thenthe high-impedance feedback for the inverters 21, 24, 27 results in avoltage of 1.25 V being produced at their inputs and outputs. Theinverters 21, 24, 27 thus operate as fast amplifiers. When voltagefluctuations occur at the input of the first inverter 21 as aconsequence of noise effects, or initial oscillations of the oscillationcrystal 10 occur, then these are amplified and are passed to the nextinverter 24. If, for example, voltage variations of 10 mV occur at theinput of an inverter 21, 24, 27, then these are typically amplified toabout 100 mV. The gain is limited by the logic voltage level, whichtypically at the same time represents the supply voltage for theinverters 21, 24, 27 as well as earth. The detection unit 2 thus emits adigital clock signal 2 a, which is produced on the basis of the analogueoutput signal from the oscillating crystal 10.

The first inverter 21 is preferably in the form of a Schmitt trigger,which amplifies voltage oscillations at the input only above anadjustable threshold value, and preferably has hysteresis in its gainresponse.

The detection unit 2 thus detects noise effects and oscillatoroscillations 1 b which occur in the output signal from the oscillatingcrystal 10, and uses them to form a digital clock signal 2 a, which isemitted at the output of the detection unit 2.

The digital clock signal 2 a is passed to a counter 4 and to the pulsegenerator 3.

The pulse generator 3 contains a univibrator 31, 32, which comprises aninverter (flank detector) 31 and a differentiating circuit 32 with whoseaid short pulses 3 b are produced from the digital clock signal 2 a.When a logic 1 is applied to the input of the pulse generator 3, then alogic 0 is produced at the output of the inverter 31. A logic 0 and alogic 1 are thus applied to the inputs of the NOR gate 32, so that theNOR gate 32 emits a logic 0. When the digital clock signal 2 a changesto the logic value 0, then a logic 0 is first of all still produced atthe output of the inverter 31 (owing to the gate delay time in theinverter 31). A logic 0 is thus applied to both inputs of the NOR gate32, and the NOR gate 32 emits a logic 1. After the gate delay time ofthe inverter 31, its output changes to logic 1, and the output of theNOR gate 32 changes back to the logic value 0. The NOR gate 32 thusemits short pulses, whose duration depends on the signal delay time inthe delay path. In addition, the inverter 31 is preceded by an RCelement 33, which delays the switching of the inverter 31 and furtherextends the pulse duration at the output of the NOR gate 32.

The output of the NOR gate 32 is connected to one input of a NAND gate34. The second input of the NAND gate 34 is coupled to a supply line 4a. When the enable signal (logic 1) is applied to the supply line 4 a,the pulses which are produced at the output of the NOR gate 32 arepassed through the NAND gate 34. The output of the NAND gate 34 changesto the logic value 0 during the pulse duration, and thus opens the gateof a PMOS transistor 35. In consequence, excitation pulses 3 b aregenerated and are emitted via the supply line 3 a to the oscillator unit1 and/or to the oscillating crystal 10. The signal delay time throughthe detection unit 2 and the pulse generator 3 is governed only by thegate delay times and is thus negligible in comparison to the oscillationduration of the oscillating crystal 10, so that the excitation pulses 3b which are emitted to the oscillating crystal are emitted synchronouslyin phase with respect to the detected oscillation 1 c of the oscillatingcrystal 10, so that additional charges are driven into the crystaloscillator 10. Noise effects and initial oscillations of the oscillatingcrystal 10 are amplified instantaneously, and thus in the correct phase.

In the present exemplary embodiment, the counter 4 has fiveseries-connected toggle flipflops 41, 42, 43, 44, 45, which areconnected such that they divide the respectively applied clock signal(input C) by a factor of 2. The counter 4 can thus assume 2⁵=32different states. Each flipflop 41, 42, 43, 44, 45 has a Set-S and aReset-R input, and the output Q is fed back to the respective data inputD. The digital clock signal 2 a is applied to the clock input C of thefirst flipflop 41. The output of the flipflops 41, 42, 43, 44 is appliedto the clock input C of the respective subsequent flipflop 42, 43, 44,45. Furthermore, the output signal Q from the four “higher-value”flipflops 42, 43, 44, 45 is supplied to a NOR gate 46. When a logic 0 isapplied to all four inputs of the NOR gate 46, then the enable signal issupplied to the pulse generator 3, in the form of a logic 1 via thesupply line 4 a. The enable signal is thus emitted to the pulsegenerator 3 only during the count states 30 and 31 of the counter 4.When the enable signal is not applied to the supply line 4 a and a logic0 is produced there, then the NAND gate 34 blocks the pulses emittedfrom the NOR gate 32, and no excitation pulses are generated and fed tothe oscillating crystal 10. This design of the counter 4 produces threeexcitation pulses (on changing to the count states 30, 31 and 0) forevery 32 detected oscillator oscillations 1 b in the output signal fromthe oscillating crystal 10. However, the design of the counter 4 can bechanged as required, so that different excitation pulse sequences areemitted to the oscillating crystal 10. For example, the outputs of theflipflops 41, 42, 43, 44, 45 can be combined as required at the input ofthe NOR gate 46 in order to emit the desired number and sequence ofpulse sequences for each counter run to the oscillating crystal 10. Itis also feasible for the number of flipflops 41, 42, 43, 44, 45 to bevaried in order to increase or to reduce the number of states of thecounter 4. Furthermore, it is also possible to use more complexcounters.

The Set-S and Reset-R inputs of the flipflops 41, 42, 43, 44, 45 arecontrolled via external supply lines 47, 48. The counter 4 and thefeeding of the excitation pulses can thus be switched on and offexternally.

The lower part of FIG. 3 shows a simulation of the output signal 1 a(XTAL) from the oscillating crystal 10 when using the method accordingto the invention. The pulses which are emitted by the pulse generator 3on the supply line 3 a (LOAD PIN) are shown in the upper part of FIG. 3.Three excitation pulses 3 c are emitted per cycle of the counter 4, thatis to say in the present case with five flipflops 41, 42, 43, 44, 45 per2⁵=32 for oscillator oscillations 1 b detected in the output signal 1 afrom the oscillating crystal 10. This excitation pulse sequence 3 c isrepeated with each cycle of the counter 4, and is annotated by thereference symbols 3 d, 3 e, 3 f in FIG. 3. The individual excitationpulses 3 b in a pulse sequence 3 c correspond to three successivedetected oscillator oscillations 1 d of the oscillating crystal 10. Inthe present exemplary embodiment, the oscillation frequency of theoscillating crystal 10 is about 26 MHz, that is to say one countercycle, which corresponds to 32 oscillator oscillations 1 d, lasts forslightly more than 1 μs. As can be seen from the lower part of FIG. 3,the oscillation amplitude of the oscillating crystal 10 increasessignificantly in each counter cycle, that is to say for every pulsesequence 3 c, 3 d, 3 e, 3 f which is emitted, so that the oscillationamplitude of the oscillating crystal 10 has increased considerably evenafter a few microseconds and after a small number of excitation pulses 3b have been fed to the crystal oscillator 10.

An oscillation frequency of about 26 MHz corresponds to an oscillationduration of about 38 nanoseconds, while the signal delay time throughthe detection unit 2 and the pulse generator 3 is in the region of 1nanosecond. The excitation pulses are thus fed to the crystal oscillator10 with a delay which is negligible in comparison to the oscillationduration and is thus sufficiently in the correct phase.

FIG. 4 shows a comparison of the oscillation starting processes of anoscillating crystal 10 according to the prior art (upper part) and thatof an oscillating crystal 10 when using the method according to theinvention (lower part). As has already been mentioned above, theoscillator amplitude of the oscillating crystal 10 rises considerablywithin a few microseconds when using the method according to theinvention. In comparison, the oscillator amplitude of the oscillatingcrystal 10 rises only slightly in the same time period when theoscillations are started according to the prior art (the scaling of thegraph in the upper part of FIG. 4 is irrelevant in this case).

After the end of the oscillation starting process, that is to say assoon as the oscillation of the oscillating crystal 10 has risen above aspecific amplitude, the detection unit 2, the pulse generator 3 and thecounter 4 are switched off.

1. A method for assisting the oscillation starting process of anelectromechanical oscillator, comprising a feedback method with thefollowing steps: a) detecting oscillator oscillations which occur in anoutput signal from the electromechanical oscillator; b) generating anenable signal during an oscillation of the oscillator; c) generating anexcitation pulse on the basis of a detected oscillator oscillation andthe enable signal; d) feeding of the excitation pulse to theelectromechanical oscillator; and e) determining, with a counter whichuses a clock signal, the time or times at which the enable signal causesthe excitation pulse or pulses to be fed to the electromechanicaloscillator, wherein a plurality of excitation pulses are fed to theelectromechanical oscillator, by carrying out the feedback methodrepeatedly, wherein groups of excitation pulses are fed to theelectromechanical oscillator periodically, with each group comprising atleast one excitation pulse, wherein no excitation pulses are fed to theelectromechanical oscillator during the time period between feeding ofthe groups of excitation pulses, wherein the time period between feedingof the groups of excitation pulses is longer than the time periodbetween two subsequent pulses in any group of excitation pulses.
 2. Amethod according to claim 1, wherein the detection of the oscillatoroscillations comprises the following steps: amplifying oscillatoroscillations which occur in the output signal from the electromechanicaloscillator; and generating a clock signal on the basis of the amplifiedoscillator oscillations.
 3. A method according to claim 1, wherein atime at which the enable signal causes the generation of the excitationpulse is predetermined.
 4. A method according to claim 1, wherein theenable signal is produced with the aid of the clock signal.
 5. A methodaccording to claim 1, wherein a predetermined number of excitationpulses are fed to the electromechanical oscillator within one group ofexcitation pulses, with the excitation pulses in the groups beinggenerated on the basis of successive, detected oscillator oscillations.6. A method according to claim 1, wherein the oscillator oscillations ofthe electromechanical oscillator which exceed a threshold value aredetected.
 7. A method according to claim 1, wherein theelectromechanical oscillator comprises a piezoelectric crystaloscillator.
 8. An apparatus for assisting the oscillation startingprocess of an electromechanical oscillator, comprising: a detection unitfor detection of oscillator oscillations which occur in an output signalfrom the electromechanical oscillator; a unit for production of anenable signal during an oscillation of the oscillator; a pulse generatoroperable to receive the enable signal and to generate an excitationpulse on the basis of a detected oscillator oscillation which is fed tothe electromechanical oscillator on the basis of the enable signal; anda counter which uses a clock signal to determine the time or times atwhich the enable signal causes the excitation pulse or pulses to be fedto the electromechanical oscillator, wherein the unit for production ofthe enable signal is operable to cause a plurality of excitation pulsesto be fed to the electromechanical oscillator, wherein the unit forproduction of the enable signal is operable to cause groups ofexcitation pulses to be fed to the electromechanical oscillatorperiodically, with each group comprising at least one excitation pulse,wherein the unit for production of the enable signal is operable toensure that no excitation pulses are fed to the electromechanicaloscillator during the time periods between the generation of the groupsof excitation pulses wherein the time period between feeding of thegroups of excitation pulses is longer than the time period between twosubsequent pulses in any group of excitation pulses.
 9. An apparatusaccording to claim 8, wherein the detection unit is operable to amplifyoscillator oscillations which occur in the output signal from theelectromechanical oscillator, and to generate a clock signal on thebasis of the amplified oscillator oscillations.
 10. An apparatusaccording to claim 8, wherein the detection unit comprises an inverterchain and at least one of the inverters in the inverter chain isoperated at the triple point.
 11. An apparatus according to claim 10,wherein the first inverter in the inverter chain detects thoseoscillator oscillations in the electromechanical oscillator which exceeda threshold value, and this first inverter is in the form of a Schmitttrigger.
 12. An apparatus according to claim 8, wherein the pulsegenerator comprises a univibrator, and an RC element is arranged in thedelay path of the univibrator.
 13. An apparatus according to claim 8,wherein the counter is operable to produce the enable signal with theaid of the clock signal.
 14. An apparatus according to claim 8, whereinthe unit for production of the enable signal is operable to cause apredetermined number of excitation pulses to be fed to theelectromechanical oscillator within one group of excitation pulses, withthe excitation pulses in the groups being generated on the basis ofsuccessive detected oscillator oscillation.
 15. An apparatus accordingto claim 8, wherein the electromechanical oscillator comprises apiezoelectric crystal oscillator.
 16. An oscillator circuit comprising:an oscillator unit with an electromechanical oscillator configured toprovide an output signal; a detection unit configured to detectoscillator oscillations which occur in the output signal from theelectromechanical oscillator; a unit configured to produce an enablesignal during an oscillation of the oscillator; pulse generatorconfigured to receive the enable signal and generate an excitation pulsebased on a detected oscillator oscillation which is fed to theelectromechanical oscillator based on the enable signal; and a counterwhich uses a clock signal to determine the time or times at which theenable signal causes the excitation pulse or pulses to be fed to theelectromechanical oscillator, wherein the unit configured to produce theenable signal is operable to cause a plurality of excitation pulses tobe fed to the electromechanical oscillator, wherein the unit configuredto produce the enable signal is operable to cause groups of excitationpulses to be fed to the electromechanical oscillator periodically, witheach group comprising at least one excitation pulse, wherein the unitconfigured to produce the enable signal is operable to ensure that noexcitation pulses are fed to the electromechanical oscillator during thetime periods between the generation of the groups of excitation pulses,wherein the time period between feeding of the groups of excitationpulses is longer than the time period between two subsequent pulses inany group of excitation pulses.
 17. An oscillator circuit according toclaim 16, wherein the oscillator unit is in the form of a Colpitzoscillator.
 18. A method for assisting the oscillation starting processof an electromechanical oscillator, comprising a feedback methodcomprising: a) detecting oscillator oscillations which occur in anoutput signal from the electromechanical oscillator; b) generating anexcitation pulse based on a detected oscillator oscillation; c) feedingof the excitation pulse to the electromechanical oscillatorsynchronously in phase with respect to the detected oscillatoroscillation; and d) determining, with a counter which uses a clocksignal, the time or times at which the excitation pulse or pulses is fedto the electromechanical oscillator, wherein a plurality of excitationpulses are fed to the electromechanical oscillator, by carrying out thefeedback method repeatedly, wherein groups of excitation pulses are fedto the electromechanical oscillator periodically, with each groupcomprising at least one excitation pulse, wherein no excitation pulsesare fed to the electrotmechanical oscillator during the time periodbetween feeding of the groups of excitation pulses, wherein the timeperiod between feeding of the groups of excitation pulses is longer thanthe time period between two subsequent pulses in any group of excitationpulses.
 19. An apparatus configured to assist the oscillation startingprocess of an electromechanical oscillator, comprising: a detection unitconfigured to detect oscillator oscillations which occur in an outputsignal from the electromechanical oscillator; a pulse generatorconfigured to generate an excitation pulse based on a detectedoscillator oscillation which is fed to the electromechanical oscillatorsynchronously in phase with respect to the detected oscillatoroscillation; and a counter which uses a clock signal to determine thetime or times at which the excitation pulse or pulses is fed to theelectromechanical oscillator, wherein a plurality of excitation pulsesare fed to the electromechanical oscillator, wherein groups ofexcitation pulses are fed to the electromechanical oscillatorperiodically, with each group comprising at least one excitation pulse,wherein no excitation pulses are fed to the electromechanical oscillatorduring the time period between feedine of the groups of excitationpulses wherein the time period between feeding of the groups ofexcitation pulses is longer than the time period between two subsequentpulses in any group of excitation pulses.
 20. A method for assisting theoscillation starting process of an electromechanical oscillator,comprising a feedback method comprising: a) detecting oscillatoroscillations which occur in an output signal from the electromechanicaloscillator; b) generating an enable signal during an oscillation of theoscillator; c) generating an excitation pulse sequence having apre-determined number of excitation pulses based on a detectedoscillator oscillation and the enable signal; and d) feeding theexcitation pulse sequence to the electromechanical oscillator, wherein aplurality of excitation pulses are fed to the electromechanicaloscillator, by carrying out the feedback method repeatedly, whereingroups of excitation pulses are fed to the electromechanical oscillatorperiodically, with each group comprising at least one excitation pulse,wherein no excitation pulses are fed to the electromechanical oscillatorduring the time period between feeding of the groups of excitationpulses, wherein the time period between feeding of the groups ofexcitation pulses is longer than the time period between two subsequentpulses in any group of excitation pulses.